1. Field of the Invention
The present invention generally relates to the measurement of absolute gravity and, more particularly, is concerned with an absolute gravity inline measuring apparatus incorporating improved operating features for attaining higher precision and accuracy.
2. Description of the Prior Art
Measurement of the acceleration of gravity has long been a matter of scientific interest. Measurements of gravity have been a valuable tool for metrologists, geophysicists, geologists and geodesists. A primary requirement for reliable gravity measurement is the longterm stability of the gravity measuring apparatus. An absolute gravity measuring apparatus developed at the Joint Institute for Laboratory Astrophysics (JILA) in Boulder, Colo., USA, employing free-fall and laser interferometric techniques holds the greatest promise for meeting this primary requirement. The JILA absolute gravity measuring apparatus is described and illustrated in an article entitled "g - the acceleration of gravity; its measurement and its importance" by Iginio Marson and James E. Faller, appearing in J. Phys. E: Sci. Instrum. 19 (1986), pages 2214 32. The JILA absolute gravity measuring apparatus is also mentioned in an article entitled "Ballistic Methods of Measuring g - the Direct Free-Fall and Symmetrical Rise-and-Fall Methods Compared" by James E. Faller and Iginio Marson, appearing in Metrolonia 25 (1988), pages 49-55. For purposes of brevity and clarity, the JILA absolute gravity measuring apparatus hereinafter will be identified as the JILAmeasuring apparatus.
The JILA measuring apparatus is transportable and basically includes an interferometer arrangement, test mass tracking chamber (also known as a dropping chamber), and a superspring mechanism. The interferometer arrangement contains a laser for generating a laser beam and optic elements for splitting, directing, and recombining the laser beam in the manner of a Michelson interferometer. The laser and optic elements of the interferometer are arranged to provide a light beam path having a pair of substantially parallel, horizontally-spaced, vertically-extending fixed and variable legs. The variable leg of the interferometer is terminated by a corner cube retroreflector which, as part of a test mass, is dropped and allowed to be freely accelerated by the Earth's gravity so as to vary the length of the light beam path in the variable leg. The times of occurrence of interferometer fringes produced by light beams recombined from the fixed and variable legs are measured and used to calculate the acceleration of the falling mass. The stabilized laser, used as the light beam source in the interferometer, provides the length standard while an atomic frequency standard provides the time standard.
The test mass tracking chamber of the JILA measuring apparatus is an elongated housing which includes a servo-controlled elevator-type cart which releasably supports the test mass therein and is vertically movable within the housing, a drag-free enclosure integral to the cart surrounding the test mass, and a drive mechanism for causing the cart to move vertically within the housing. In a test mass drop mode, the cart is caused to accelerate away from and thus "drop" the test mass and thereafter to track the free falling test mass, without touching it, while measurement of the falling test mass are being taken. The test mass initially rests on kinematic mounts in the elevator-type cart. The cart can be driven upwardly and downwardly along vertical guide rails within the evacuated housing by a thin stainless steel belt connected to a DC motor of the drive mechanism. After being dropped, the position of the falling test mass relative to the cart is monitored by focussing light from a light-emitting diode carried on the cart, through a lens attached to the falling test mass, onto a position-sensitive photodetector also carried on the cart. An error signal thus derived is used to control the drive motor to accelerate the cart downwardly along a drop path so as to drop the test mass and then leave the test mass falling freely inside the descending cart. As it approaches the bottom of the drop path, the descending cart is controlled to slow down and then gently arrest the fall of the dropped test mass. The cart, with the test mass supported thereon, can then be driven upwardly to return the test mass to the top of the drop path for initiating the next drop and measurement. This rapid turnaround capability is primarily responsible for the ability of the system to acquire data at a very high rate, such as one measurement every two seconds.
The superspring mechanism of the JILA measuring apparatus includes a plurality of auxiliary springs supporting a platform from a base portion of the apparatus, a superspring inertial mass connected with a transparent sphere, and a main spring suspending the inertial mass from the platform. The inertial mass also has a cornercube retroreflector which terminates the fixed leg of the interferometer retroreflector. The superspring mechanism functions to maintain the test mass and the retroreflector thereof motionless in the fixed leg of the interferometer as the test mass is dropped and allowed to be freely accelerated by the Earth's gravity. The superspring mechanism also includes a photocell and a light source mounted on the platform on opposite sides of the transparent sphere. Light is transmitted from the light source through the sphere to the photodetector. An electro-mechanical servo system is connected to the photocell and, in response to the transmitted light, is operable to drive the platform in order to set the effective lengths of the auxiliary springs in response to any motion of the inertial mass, retroreflector, and transparent sphere in order to cancel these motions and maintain the inertial mass in a motionless state and thereby maintain the fixed leg of the interferometer at a fixed beam length.
The three primary features of the above-described JILA measuring apparatus which account for its ability to achieve high precision and accuracy without sacrificing small size and hence transportability are the test mass tracking chamber, the interferometer, the superspring mechanism. The laser of the interferometer provides high accuracy, the tracking chamber eliminates several sources of systematic errors while providing a rapid means of repeatedly releasing the test mass. The long-period isolation provided by the superspring mechanism greatly decreases sensitivity of the JILA measuring apparatus to ground vibrations. This avoids large drop-to-drop scatter as well as possible systematic recoil effects that might otherwise arise from the finite size of the apparatus.
The fundamental problem involved in absolute gravity measurements is the recognition and elimination of sources of systematic errors in order to obtain accurate measurements as opposed to obtaining consistent but inaccurate measurements. Although the JILA measuring apparatus eliminates most sources of systematic errors, others are known to still exist which degrade the precision and accuracy of the apparatus. Consequently, a need still remains for improvements in the design of the JILA absolute gravity measuring apparatus to attain enhanced performance.